1. Field of the Invention
The present invention relates to a force measuring apparatus, particularly a balance. The force measuring apparatus includes a parallelogram guide means with a fixedly supported parallelogram side member, two parallel guide members which are longitudinally stiff and bending-elastic in the plane of the parallelogram, and an additional parallelogram side member for receiving the force to be measured. The additional parallelogram side member is guided so as to be deflectable in parallel on the fixedly supported parallelogram side member by means of the guide members. The apparatus further includes a measuring transformer for supplying a signal corresponding to the force to be measured and a device serving for transmitting at least a portion of the force on the measuring transformer to be measured which acts on the deflectably guided parallelogram side member, wherein this device is coupled by means of a coupling member to the deflectable parallelogram-side member, and wherein the coupling member is longitudinally stiff and bending-elastic in the plane of the parallelogram and the coupling member includes a localized area which is bending-elastic transversely of the plane of the parallelogram. The present invention further relates to a method of manufacturing such a force measuring apparatus.
2. Description of the Related Art
In a balance of the above-described type known from U.S. Pat. No. 4,799,561, whose measuring transformer is based on the known principle of electromagnetic force compensation and whose force transmitting device is formed by a lever which extends between the coupling member and the moving coil of the electromagnetic force compensation system, measurement errors are to be avoided by means of the area of the coupling member which is bending-elastic transversely of the parallelogram plane, wherein the measuring errors are caused by a torque produced by the load to be weighed in the plane extending perpendicularly to the plane of the parallelogram. These torques are produced when the load to be weighed is placed on a weighing dish connected to the deflectably guided parallelogram side member laterally offset from the center of the deflectably guided parallelogram side member in the plane extending perpendicularly to the plane of the parallelogram. Twisting of the parallelogram guide means caused by this torque in the plane perpendicular to the plane of the parallelogram, particularly of the deflectably guided parallelogram side member receiving the weight force to be measured, is to be absorbed in the known balance by the area of the coupling member which is bending-elastic transversely of the plane of the parallelogram and, as a result, a transmission of the twisting to the lever and to the measuring transformer is to be avoided. Although one could assume that this known measure solves the problem of lateral torques which act on the parallelogram guide means and depend on the load application, however, upon closer examination, it has been found that, for the purposes of high resolution balances, it is not possible to sufficiently suppress off-center load errors by merely providing at any location a localized area which is bending-elastic transversely of the plane of the parallelogram.
A coupling member of a different type which is bending-elastic in the plane of the parallelogram as well as transversely of the plane of the parallelogram is known from CH-A5-652 207. In this coupling member, the bending elasticities transversely of the plane of the parallelogram and in the plane of the parallelogram are localized immediately next to each other at each end portion of the coupling member, while the portion of the coupling member between these two pairs of bending elasticities at the end portions is stiff with respect to bending and has a substantially greater material thickness than the end portions. However, particularly in high resolution balances, it is not possible to sufficiently suppress the off-center load errors by merely arranging at the ends of the coupling member two localized areas which are bending-elastic transversely of the plane of the parallelogram.